Core-crush resistant fabric and prepreg for fiber reinforced composite sandwich structures

ABSTRACT

A core crush resistant prepreg for use in making a fiber reinforced composite panel structure is provided. The prepreg comprises a woven fabric consisting essentially of carbon fiber tow strands impregnated with a hardenable polymeric resin composition. Typically the fabric has an areal weight of from about 180 to about 205 grams per square meter. The prepreg has an average fiber tow aspect ratio of less than about 15.4, a prepreg thickness of at least about 0.245 mm, and a prepreg openness of at least about 1.2 percent but less than about 6.0 percent. Preferably, the resin composition is predominantly viscous in nature and has a tan δ value of between 0.9 and 2.0 at an elevated temperature between 70° C. and 140° C., and an average epoxy functionality of greater than 2.0. A method for evaluating core crush resistance properties of a prepreg is also provided. The method includes determining a fiber tow average aspect ratio of the prepreg, determining a prepreg thickness, and comparing said average fiber tow aspect ratio and prepreg thickness to a set of predetermined values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.09/273,637, filed Mar. 23, 1999 now U.S. Pat. No. 6,261,675.

FIELD OF THE INVENTION

The invention relates to composite sandwich structures, preferablyhoneycomb core, composite sandwich structures, and to fabric and prepregcomponents for such composite structures. More particularly, theinvention relates to core crush resistant, honeycomb core compositesandwich structures, particularly those composite structures used in theaerospace industry, and to fabrics and prepregs for making suchcomposite sandwich structures.

BACKGROUND OF THE INVENTION

Honeycomb core composite sandwich structures find widespread use in theaerospace industry as panel components in various aerospace structures.The honeycomb core composites are formed from a lay-up of prepreg skinplies encompassing a honeycomb core, the latter typically having bevelededges. The prepreg plies may be fabrics, tapes, or non-wovens that havebeen pre-impregnated with a thermosetting, thermoplastic or otherpolymeric resin. The fabrics used to form the prepregs are wovenfabrics, formed primarily or entirely of high modulus, reinforcingfibers in the form of continuous filament tows. Curing of the lay-up iscarried out in a high temperature, high pressure environment, typicallyin an autoclave.

The technical requirements of aerospace end uses generally dictate thatthe prepregs and prepreg components meet a rigid set of chemical,physical, and mechanical specifications including overall prepreg basisweight, fiber modulus, and resin flow rate. The basis weight of theprepreg and the high strength properties of the fibers and the resin, incombination with the strength properties of the honeycomb corecomponent, impart high strength-to-weight, and high stiffness-to-weightratios to the final composite structure. In addition, the flow ratecharacteristics of the resin and the high pressures used to cure thecomposite, minimize porosity, i.e., the inclusion of voids and throughholes, that might impair strength, the desired impervious nature, and/orsurface smoothness of the final honeycomb panel sandwich structure.

Even though honeycomb core composite panels have long been used in theaerospace industry, manufacture of these structures is still plagued byhigh reject scrap levels, generating substantial quantities of unusablescrap and impacting negatively on manufacturing economics. Partialcollapse of the honeycomb core during curing of the composite, known inthe industry as “core crush”, is a particularly common reason forrejection of cured panels. Core crush is typically observed in thebeveled edge or chamfer region of the honeycomb structural part.

Substantial effort and research extending over many years have beendirected to the core crush problem. For example, U.S. Pat. No. 5,685,940to Hopkins discloses an improved tiedown method to produce or preventcore crush and ply wrinkling in honeycomb sandwich structures. Ascrim-supported barrier film is placed between the fiber-reinforcedresin composite laminates and honeycomb core to prevent resin flow fromthe prepreg into the honeycomb core. A tiedown ply between the core andthe barrier film is used to reduce slippage of the barrier film relativeto the core during curing. In addition, a film adhesive having a curingtemperature lower than that of the laminate resin is placed between thetiedown plies just outside the net trim line. During the curing process,cured film adhesive bonds the tiedown plies to one another before thecuring of the prepreg laminates, thus strengthening the tiedown andreducing core crush. The Hopkins patent also discusses other methods andstructural modifications which have been proposed for minimizing oreliminating the core crush. Nevertheless, core crush remains asignificant problem in the industry.

SUMMARY OF THE INVENTION

The present invention provides a core crush resistant prepreg for use inmaking a fiber reinforced composite sandwich structure. Use of theprepreg of this invention, can significantly reduce the degree of corecrush as compared to conventional structures.

In accordance with a first aspect of the invention, it has been foundthat the core crush problem associated with honeycomb core compositesandwich structures can be significantly reduced by controllingconstruction of the fabric used to prepare the prepreg. In particular,it has been found that core crush can be substantially reduced bycontrolling the cross sectional aspect ratio of the carbon fiber tow inthe prepreg, the average thickness of the prepreg and the openness ofthe prepreg, as measured by visual inspection. In particular, prepregsaccording to the invention are made from fabrics having an areal weightrange of from about 150 to 400 grams per square meter. The prepreg hasan average tow aspect ratio of less than about 15.5, an average prepregthickness of at least about 0.245 mm, and/or an openness of at leastabout 1.2% but less than about 10.0%.

While not wishing to be bound by theory, it is believed that average towaspect ratio, prepreg openness and prepreg thickness determine thefrictional force between prepreg plies during the curing step in themanufacture of honeycomb core composite sandwich structures. Whenprepreg properties of tow aspect ratio, prepreg thickness and prepregopenness are maintained within the ranges set forth above, sufficientfrictional force is provided between prepreg plies such that theinnermost prepreg plies, adjacent the honeycomb core, are restrainedfrom slipping during the curing process to thereby eliminate or minimizecore crush.

It has also been found, according to the present invention, that whenthe tow aspect ratio, prepreg thickness and prepreg openness areoptimized to minimize core crush, the porosity of the final honeycombcore composite sandwich structure can be unacceptable. The porosityproblems can be especially prevalent in thicker composite sandwichstructures and especially when a low flow resin system is used toimpregnate the prepreg. The present invention employs a hardenablepolymeric resin composition having a flow rate higher than the flow rateof resins traditionally used in commercial practice in the aerospaceindustry in prepregs for honeycomb core composite sandwich structures inorder to maintain acceptable porosity in the final composite structure.Therefore, prepregs according to the present invention are impregnatedwith a hardenable polymeric resin composition having rheology which ispredominately viscous in nature, such that the ratio of viscous toelastic components of the viscosity, i.e., tan δ, is within thefollowing defined ranges.

Prior to significant resin cross-linking or curing, the resincomposition used in this invention preferably has a tan δ of betweenabout 1.2 and about 2.0, preferably between about 1.5 and about 1.8,more preferably about 1.35, at 70° C.; or, a tan δ of between about 0.7and about 2.0, preferably between about 0.9 and about 1.8, morepreferably about 1.35, at 100° C.; or, a tan δ of between about 0.5 andabout 1.7, preferably between about 0.7 and about 1.5, more preferablyabout 1.35, at 140° C.

Preferably, the tan δ of the resin composition is from about 0.5 toabout 2.0, more preferably between about 1.0 and about 1.8, mostpreferably about 1.35, throughout the elevated temperature range of fromabout 70° C. to about 140° C., or if the minimum viscosity temperatureis below 140° C., the range of from about 70° C. to the minimumviscosity temperature.

More preferably, prior to significant resin cross-linking or curing, theresin composition has a tan δ of between about 1.0 and about 2.0, morepreferably between about 1.2 and about 1.8 at about 70° C.; betweenabout 0.7 and about 2.0, more preferably between about 1.0 and about 1.7at about 100° C.; and, between about 0.5 and about 2.0, more preferablybetween about 0.6 and about 1.7 at about 140° C., or at the minimumviscosity temperature, if the minimum viscosity temperature is below140° C.

Preferably, the resin composition comprises an epoxy resin and has anaverage epoxy functionality of greater than 2.0.

According to another aspect of the invention, a prepreg for attaininggood core crush performance while minimizing porosity in honeycomb corecomposite sandwich structures is provided having different tow aspectratios in the warp and weft directions. In particular, prior artprepregs have been prepared using identical carbon fiber tows as warpand weft components. In accordance with this aspect of the invention, ithas been found that different carbon fiber tow constructions providedifferent tow cross-sectional aspect ratios. By using different tows toform the warp and weft components of the fabric component of theprepreg, it is possible to optimize and balance the frictionalproperties of the prepreg, thus minimizing the core crush properties ofthe prepreg, while also minimizing any substantial porosity increase inthe final composite structure. In accordance with this aspect of theinvention, the prepreg is formed from different carbon fiber tows in thewarp and weft directions. The prepreg has a tow aspect ratio of nogreater than about 13.0 in one of the warp and weft directions and a towaspect ratio of at least about 13.5 in the other of the warp and weftdirections. In addition, the prepreg has an openness of no greater thanabout 5.0%. Preferably the prepreg has a prepreg thickness of from about0.250 mm to about 0.275 mm. Prepregs according to this aspect of theinvention, i.e., fabrics having hybrid tow aspect ratios, are wellsuited for use with a wide variety of resins systems, e.g. of varyingrheologies.

In accordance with yet another embodiment of the present invention,there is provided a prepreg which comprises in the resin-impregnatedfabric, tow strands having a predetermined substantially stable,non-round cross sectional shape. Preferably, such tow strands have anaverage fiber tow aspect ratio of from about 12.0 to about 14.5, and aprepreg openness of no great than about 5.0%. It has been discoveredthat such prepregs can substantially reduce both core crush and porosityin fiber reinforced composite structures.

Thus, the prepregs of the present invention are capable of substantiallyreducing core crush and porosity in honeycomb core composite structures.However, no complexities are added to the lay-up and/or curing processfor preparing the panels. Moreover, the present invention does notintroduce abnormal or unusual components into the composite structure.Thus, the present invention can provide substantially reduced core crushand substantially reduced reject levels without requiring substantialmodification to the conventional procedures and/or lay-up arrangementused to prepare honeycomb core composite sandwich structures. Inaddition, because various fiber tows can be manipulated in accordancewith the present invention to impart desirable prepreg properties so asto reduce core crush, this invention allows prepreg manufacturersgreater freedom in selecting fiber tows or fabrics for use in preparingprepregs, and thus less dependence on specific specialty fiber towmanufacturers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a woven fabric for use in the prepreg of thisinvention;

FIG. 2 illustrates a prepreg prepared from the woven fabric of FIG. 1 byresin impregnation thereof;

FIG. 2A is a diagrams illustrating the cross section of the prepreg ofFIG. 2 taken along the line 2A—2A thereof and illustrates the thicknessof the prepreg and the aspect ratio of the tows along one direction ofthe prepreg;

FIG. 2B illustrates the cross section of a prepreg taken along thecenter line of a tow strand along one direction of a prepregdemonstrating the thickness of the prepreg and the predeterminedsubstantially stable round cross sectional shape as well as the towaspect ratio of the tow strands along the other direction of theprepreg.

FIG. 2C illustrates the cross section of a prepreg taken along thecenter line of a tow strand along one direction of a prepregdemonstrating the thickness of the prepreg and the predeterminedsubstantially stable non-round cross sectional shape as well as the towaspect ratio of the tow strands along the other direction of theprepreg;

FIG. 3 illustrates a woven fabric for use in the prepreg of thisinvention in which the weft tows have a different tow construction andaspect ratio as compared to the warp tows;

FIG. 4 illustrates a prepreg made from the woven fabric of FIG. 3;

FIG. 4A illustrates the cross section of the prepreg of FIG. 4 takenalong the line 4A—4A and illustrates the tow aspect ratio in a firstdirection;

FIG. 4B illustrates the cross section of the prepreg of FIG. 4 takenalong the line 4B—4B and illustrates the tow aspect ratio in the otherdirection;

FIG. 5 is a graph illustrating the correlation between prepreg thicknessand core crush degree in honeycomb core composite sandwich structures;

FIG. 6 is a graph illustrating the correlation between prepreg averagefiber tow aspect ratio and core crush degree in honeycomb compositesandwich structures;

FIG. 7 is a diagram of a standard core crush panel used for testing thecore crush properties of prepregs of this invention;

FIG. 7A is a cross sectional view of the core crush panel of FIG. 7taken along the line A—A and illustrates the lay-up of the prepreg pliesand the honeycomb core before curing; and

FIG. 8 is a diagram illustrating the determination of the core crusharea in a standard core crush panel.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Illustrated in FIG. 1 is a woven fabric 10 that can be used to preparethe prepreg of this invention. Although other conventional fabrics suchas knitted fabric and nonwoven fabric can be used, a woven fabric ispreferred. Furthermore, a variety of tow orientations, e.g., ±45°,0°/90°, etc., may be used to weave suitable fabrics for the presentinvention. Typically, a two-dimensional biaxially woven fabric ispreferred. The woven fabric 10 has a plurality of warp tow strands 12interweaving with a plurality of weft tow strands 14. The term “tow” and“tow strand” are used herein interchangeably to refer to the yarn usedto form a woven fabric. As will be apparent from the description below,each tow or tow strand is essentially a bundle of a plurality of fiberfilaments. As used herein, “warp” and “weft” are used herein to refer tothe two different directions in which the tow strands are oriented in atwo-dimensional woven fabric. Although in the textile art, warp towstypically are referred to yarns that run parallel to the selvage orlonger dimension of a fabric, the term “warp” as used herein can meaneither of the two directions, and “weft” the other direction.

As illustrated in fabric 10, a plurality of openings 16 are formed bythe interweaving warp tow strands 12 and weft tow strands 14. Typically,the openings 16 are square or rectangular openings, i.e., the warps andwefts cross at a substantially right angle. Although other fabricpatterns such as basket weave, crow foot satin weave, leno weave, mockleno weave, or unidirectional weave can be used, satin weave, twillweave, and plain weave are most preferred for the prepreg of the presentinvention. As is well known in the art, in a plain weave pattern asillustrated in FIG. 1, each weft tow passes successively over and undereach single warp yarn in alternating rows. In a satin weave pattern,warp tow strands outnumber the weft tow strands, and are characterizedby floats that run in the warp direction on the face in such a manner asto reflect light, producing gloss, luster, or shine. Typically, eachweft tow floats over two or more warp tows. In twill weaves, tows areinterlaced and dominant diagonally lined patterns are created.Typically, a series of floats are staggered in a definite pattern in thewarp direction, either to the left (left-hand twill), to the right(right-hand twill), or equally to the left and right in a zigzag effect(broken twill), which produces no diagonal lines.

Each tow strand 12 and 14 is formed from a plurality of continuousfilaments as will be apparent. The term “filament” and “fiber” are usedherein interchangeably to mean an individual fiber or filament of themulti-fiber or multi-filament tow. Various different high modulusreinforcing fibers can be used to form the tows such as carbon fibers,fiberglass, or aramid. Preferably, carbon fibers are utilized. This isbecause generally they have desirable properties of high tensilestrength, high modulus, and low density and good resistance to adverseenvironmental factors such as high temperature, high moisture and highacidity. As is known in the art, carbon fibers are generally made byconverting various precursor organic polymeric fibrous materials to acarbonaceous form under high temperature while retaining the originalfibrous configuration essentially intact except that the fiber crosssectional size is normally decreased and/or the fiber cross-sectionalshape can be changed because of stretching during carbonizationprocessing. Generally, for purposes of this invention, carbonization canbe conducted either before or after the precursor filaments are bundledtogether to form a tow.

As used herein, “carbon” fibers or “carbonaceous” fibers generally referto graphite fibers and amorphous carbon fibers. Graphite fibers consistessentially of carbon and have a predominate x-ray diffraction patterncharacteristic of graphite. Amorphous carbon fibers ordinarily refer tofibers consisting essentially of carbon and exhibiting an essentiallyamorphous x-ray diffraction pattern.

The carbon fibers typically used for the tow strands are structuraltextile fibers and have a standard tensile modulus, or Young's modulusof above about 10×10⁶ psi, e.g., from about 10×10⁶ psi to about 120×10⁶psi, preferably from about 20×10⁶ psi to about 100×10⁶ psi, morepreferably from about 25×10⁶ psi to about 75×10⁶ psi, and mostpreferably from about 30×10⁶ psi to about 45×10 psi. Suitable carbonfibers should have an individual denier of from about 0.2 to about 1.0g/9000 m, preferably from about 0.3 to about 0.8 g/9000 m, morepreferably from about 0.4 to about 0.7 g/9000 m, and most preferablyfrom about 0.55 to about 0.65 g/9000 m. The diameter of each singlecarbon fiber filament can be in the range of from about 0.5 to about 50μm, preferably from about 1 to about 25 μm, more preferably from about 5to about 15 μm, and most preferably about 10 μm.

Each carbon fiber tow strand can have a total filament count of fromabout 1,000 to about 80,000, preferably from about 2,000 to about50,000, more preferably from about 3000 to about 18,000, and mostpreferably 3,000 to about 12,000. For example, a typical filament countof a fiber tow strand can be 3,000, 6,000, 12,000, from about 1,000 toless than about 3,000, or can be greater than 3,000 but no greater than18,000. Typically, the fabric used in the prepreg of this inventionshould have an areal weight of from about 150 to about 400 grams persquare meter, preferably from about 160 to about 250 grams per squaremeter, more preferably from about 180 to 205 grams per square meter, andmost preferably from about 185 to about 201 grams per square meter. Asapparent to a skilled artisan, a fabric with an areal weight withinthese ranges is particularly suitable in making fiber reinforcedcomposite structures for aviation end uses.

In order to make the prepreg of the present invention, the fabric asdescribed above is impregnated with a polymeric matrix resincomposition. Generally any resin compositions used in the art forimpregnating prepregs used for making fiber reinforced compositestructures can be suitably used in the present invention. For example,such suitable resin compositions are disclosed in e.g. U.S. Pat. No.4,599,413 to Moulton et al., U.S. Pat. No. 5,626,916 to Kishi et al.,and European Patent Application EP 0819723 Al to Kishi et al., each ofwhich are incorporated herein by reference.

Typically, a suitable resin composition comprises from about 40% toabout 95% by weight of polymeric matrix resin, from about 5% to about40% by weight of curing agent, and from about 0% to about 20% by weightof a flow control agent. Many different polymeric matrix resins known inthe art can be used in the present invention. Examples of suitablepolymeric matrix resin include, but are not limited to, epoxy resins,phenolic resins, polyester resins, polyimide resins, polybenzimidazoles,polyurethanes, vinyl ester resins, bismaleimide resins, polyetherimide,polyetherketone, polyamides, etc. Polymeric matrix resins are processedin a variety of ways such as hardening a liquid, melting, foaming, andsolvent activation. However, a thermosetting epoxy resin is mostpreferred, especially epoxy resins derived from amines, phenols or vinylcompounds. As is known in the art, epoxy resins utilize epoxide groupsas the functional groups in the curing reaction. Suitable thermosettingepoxy resins may include, but are not limited to,diglycidyl-p-aminophenol, triglycidyl aminocresol,triglycidyl-p-aminophenol, tetraglycidyl diaminodiphenylmethane,tetraglycidyl ethers of methylenedianiline, bisphenol A type epoxyresins such as diglycidyl ethers of bisphenol A, bisphenol F type epoxyresins such as diglycidyl ethers of bisphenol F, bisphenol S type epoxyresins, phenol novolac type epoxy resins, cresol novolac type epoxyresins, resorcinol type epoxy resins, epoxy resins with a naphthaleneskeleton, biphenyl type epoxy resins, dicyclopentadiene type epoxyresins and diphenylfluorene type epoxy resins, etc. These resins can beused individually or in any appropriate combinations. Preferred epoxyresins are those having an epoxy functionality of at least 2.0, morepreferably greater than 2, because of the technical demands imposed byaerospace end uses. In other words, bifunctional, trifunctional, orhigher functional epoxy resin are preferred. In particular,glycidylamine type epoxy resin and glycidyl ether type epoxy resinhaving a functionality of greater than 2 are preferred.

Curing agents are those compounds having an active group that can reactwith a functional group in the polymeric resin, for example, an epoxygroup of an epoxy resin. Suitable curing agents for epoxy resinsinclude, but are not limited to, diaminodiphenyl methane,diaminodiphenylsulfone, various substituted ureas, aminobenzoates,various acid anhydrides, various isomers of dicyandiamide, phenolnovolac resins, cresol novolac resins, polyphenol compounds, imidazolederivatives, alphatic amines, tetramethylguanidine, carboxylicanhydrides, carboxylic acid hydrazides, carboxylic acid amides,polymercaptan, Lewis acid complexes such as a boron trifluorideethylamine complex, and the various heterocyclic multifunctional amineadducts as disclosed in the aforesaid U.S. Pat. Nos. 4,427,802 and4,599,413. Any of the above curing agents can be used eitherindividually or in various combinations in the resin composition of thepresent invention.

As is well known in the art, flow control agents in a resin compositionare used to adjust the viscoelasticity of the resin composition. Theflow control agents used in the present invention are also oftenutilized with resin compositions to modify the mechanical performance ofthe cured composite article, e.g., by providing toughness. Suitable flowcontrol agents may include, but are limited to, e.g., solid rubbers,liquid rubbers, thermoplastic resin elastomers, organic and inorganicparticles, and short fibers. Certain thermoplastics that are soluble orpartially soluble in epoxy resins may also be used as flow controlagents. Such thermoplastics include, e.g., polysulfones, polyethersulfones, polyether imides, and polyethylene oxide. The various flowcontrol agents can be used individually or in combinations.

Typically, both a liquid rubber or elastomer and a solid rubber orelastomer, which are fully or partially soluble in the epoxy resins areincluded. As is known in the art, when solid rubbers are used in anepoxy resin composition, the temperature dependence of theviscoelasticity function of the resin composition is decreased and thesurface smoothness of the skin panel in the cured fiber reinforcedcomposite sandwich structure is increased. Preferably, the solid rubberand/or liquid rubber included in the resin composition includes one ormore functional groups such as carboxyl groups and amino groups, whichreact with the epoxy groups of an epoxy resin in the resin composition.Examples of preferred solid rubbers include, e.g., solidacrylonitrile-butadiene rubber, hydrogenated nitrile rubber, etc.Various other solid and liquid elastomers known in the art can all beused.

Particulate flow control agents that are included in the resincomposition may also function as fillers or extenders. Examples of suchan agents include, e.g., fumed silica, mica, calcium carbonate, calciumphosphate, glass, metal oxides, cellulose, starch, clays, diatomaceousearth, etc.

Other materials such as catalysts, antioxidants, chain extenders,reactive diluents, and the like all of which are known in the art, canalso be optionally included. Although any resin composition as describedabove can be useful in the present invention, it is preferred however toemploy an epoxy resin composition which exhibits significant viscousflow that is higher than that of some of the conventional resins used inprior art prepregs used for preparing composite structures for aviationuses. In particular, it is preferred to use an epoxy resin compositionwherein the elastic and viscous components of the composition are ofsimilar magnitudes. For example, the preferred epoxy resin compositionshould have a tan δ that falls within the following defined ranges,prior to significant resin cross-linking or curing:

The resin composition used in this invention preferably has a tan δ ofbetween about 1.2 and about 2.0, preferably between about 1.5 and about1.8, more preferably about 1.35, at 70° C.; or, a tan δ of between about0.7 and about 2.0, preferably between about 0.9 and about 1.8, morepreferably about 1.35, at 100° C.; or, a tan δ of between about 0.5 andabout 1.7, preferably between about 0.7 and about 1.5, more preferablyabout 1.35, at 140° C.

Preferably, the tan δ of the resin composition is from about 0.5 toabout 2.0, more preferably between about 1.0 and about 1.8, mostpreferably about 1.35, throughout the elevated temperature range of fromabout 70° C. to about 140° C., or if the minimum viscosity temperatureis below 140° C., the range of from about 70° C. to the minimumviscosity temperature.

More preferably, prior to significant resin cross-linking or curing, theresin composition has a tan δ of between about 1.0 and about 2.0, morepreferably between about 1.2 and about 1.8 at about 70° C.; betweenabout 0.7 and about 2.0, more preferably between about 1.0 and about 1.7at about 1 00° C.; and, between about 0.5 and about 2.0, more preferablybetween about 0.6 and about 1.7 at about 140° C., or at the minimumviscosity temperature, if the minimum viscosity temperature is below140° C.

Typically, the preferred resin composition used herein can have aminimum complex viscosity of 100 to 50,000 centipoise when heated at thetypical processing rates used in the industry (0.5-5° C./min). Further,it is preferred that the resin composition comprises an epoxy resin andhas an average epoxy functionality of greater than 2.0.

The value of tan δ is determined for purposes of the present inventionusing a Rheometrics Scientific RDA-II or comparable instrument, operatedin the oscillation mode using parallel plates with a diameter of 40 mmand a gap of between 0.5 and about 1 mm. Strain is set at 40% and torqueis adjusted within the range of 5 to 1200 g·cm (0.49-117.7 mN·m).Frequency is 10 rad/s (1.59 Hz). Those skilled in the art will recognizethat the critical parameters are the oscillation frequency and strain.The other parameters can be adjusted to provide identical measurementconditions using different geometries (and/or using differentinstruments which operate on the same principle). Measurements are takenwithin the temperature range of 70 to 140° C. at a heating rate of 2°C./min.

It has been discovered in accordance with the present invention that,when the prepreg of the present invention made with an epoxy resincomposition having a tan δ value within the ranges as defined above, theporosity in the finished composite structure prepared from the prepregis reduced as compared to that in a sandwich structure manufactured froma prepreg prepared with a conventional epoxy resin. As used herein,porosity refers to the percentage of the area occupied by the pores orvoids in a cross section of a finished fiber reinforced composite panelbased on the total area of the cross section. Generally, porosityincludes all pores or voids formed in cured laminates of the finishedfiber reinforced composite structure, both in the inter-ply zones, i.e.,the zones between plies, and the intra-ply zones, i.e., the zones withinone cured prepreg ply.

As will be apparent to the skilled artisan, in the resin composition ofthe present invention, the tan δ and the flow rate can be controlled bymany different methods. For example, they can be controlled by varyingthe types and relative content of the solid and/or liquid elastomers,and the extent of the epoxy-elastomer reaction, i.e., the selection ofthe types and extent of functional groups of the solid or liquidelastomers capable of reacting with the epoxy resins in the resincomposition and/or by selection of resin and curing compositions havingappropriate viscosities.

The hardenable resinous material can be applied to the fabric using anyconventional method known in the art. For example, the resin can beapplied by a solvent method or a solventless method, both of which areknown in the art. In the solvent method, which is also known as towermethod or wet method, the prepreg is prepared by impregnating the fabricwith the resin composition that is dissolved in an appropriate solvent.For example, a resin composition can be dissolved in a solvent in aresin tank and applied to a fabric as the fabric passes through thesolution in the tank. In a solventless method (also known as hot-meltmethod), the resin is formed into a resin film on an appropriatesubstrate and is subsequently transferred onto the fabric by heat andpressure. For example, a solvent free polymeric resin system is preparedand a paper substrate is coated with a resin film from the polymericresin. A prepreg is then prepared by transferring resin films from twopaper substrates onto the top and bottom surfaces of a fabric, while tworesin films sandwiching the fabric are heated and consolidated.

The resin should be applied to the fabric such that the fabric issubstantially impregnated. The resultant prepreg should have a resincontent of from about 20 to about 60 percent by weight, preferably about30 to about 50 percent by weight, more preferably about 35 to about 45percent by weight, and most preferably about 38-42 percent by weight,based on the total weight of the prepreg. The prepreg thus prepared canthen be wound onto a roll or the like for storage or shipping.Optionally, the prepreg can be subjected to various post-impregnationtreatments. For example, as is known in the art, the prepreg can bepolished (which is also known as being “calendered” or “subjected to“compaction”) to make the surface of the prepreg more even and to reducethe openness of the prepreg. Typically, a prepreg can be polished bypressing the prepreg under a pressure of about 40 to 80 psi at atemperature of about 120-160° F.

FIG. 2 illustrates a prepreg made by impregnating a fabric of FIG. 1with a resin composition. FIG. 2A is a diagram showing the crosssectional view of the prepreg of FIG. 2 taken along a weft tow strand 24(line 2A). As indicated in FIG. 2A, the prepreg has a maximum thicknessT_(p). The warp tow strand 22 has a maximum tow width W_(warp) and amaximum tow thickness of T_(warp). Likewise, the weft tow strand 24 alsohas a maximum tow width W_(weft) and a maximum tow thickness of T_(weft)(not shown). The distance o between two adjacent warp tow strands 22indicates one dimension of the opening 26 formed by two adjacent wefttow strands 24 and two adjacent warp tow strands 22.

As used herein, “tow aspect ratio” of a prepreg is defined to be theratio between the maximum cross sectional width of a tow strand, W, andthe maximum cross sectional thickness of the tow strand, T, as measuredin a prepreg, that is, W/T. It should be understood that tow aspectratio is affected by the various steps in, e.g., tow manufacturing,fabric preparation from tows, prepreg manufacturing from the fabric, thepost-impregnation treatment of the prepreg, as well as the steps in thepreparation of composite sandwich structures from the prepreg.Typically, a generally round tow (See FIG. 2B) in a prepreg will have alow tow aspect ratio. Conversely, a tow that is flattened during themanufacturing processes will have a large tow aspect ratio. As usedherein, the term “prepreg average tow aspect ratio” is the average ofthe tow aspect ratio of the warp tow strands and the weft tow strands inthe prepreg. Typically, measurements taken from at least 25 to 50different tows in each direction should be used in arriving at theaverage tow aspect ratio of a prepreg. Likewise, the term “average towaspect ratio in one direction” refers to the average tow aspect ratio ofthe tow strands in one direction of a two-dimensional woven fabric.

In accordance with this invention, different methods can be used tomanipulate or modify the cross sectional tow shape, and therefore thetow aspect ratio of a fiber tow strand of a prepreg following weaving,impregnation and optionally post-impregnation treatment. Examples ofsuch methods include, but are not limited to, sizing the tows prior toweaving, tow twisting, tow twisting and untwisting, varying the crosssectional shapes of the individual filaments in the tows and/oremploying various other modifications of the tow forming process.

It has been found that irregularities in cross sectional shapes of theindividual fibers or filaments in the tow can increase the entanglementof the filaments and reduce tow aspect ratio. In particular, filamentshaving specific cross sectional shapes can be selected to vary the towaspect ratio. For example, it has been found that individual filamentshaving a kidney or pea shape in cross section will generally form arounder fiber tow having a smaller aspect ratio following weaving andimpregnation as compared to an otherwise identical tow in which eachfilament has a round cross section.

Another method for modifying the tow construction and tow aspect ratiois by twisting and/or twisting untwisting the tow to a desired extent.Twisting processes are well known in the art. As used herein, a “twistedtow” means a tow that is subjected to a twisting process during the towforming but which is not subjected to an untwisting process as describedbelow. Typically, twisted tows have a rounder shape and thus a smallerfiber tow aspect ratio following weaving. The degree of twisting, i.e.,the number of turns per unit length, can vary with the shape, or towaspect ratio desired. For example, twisted carbon fiber tows havingabout 15 twists per meter are generally available and can be used inthis invention. An “NT” tow, that is, a tow that is never twisted, canalso be used in this invention. “Untwisted” tows or “UT” tows can beformed by first twisting a filament bundle to a desired degree and thenuntwisting, i.e., winding the twisted fiber filament bundle or tow inthe opposite direction, to a desired degree. Typically, NT tows have arelatively flatter tow shape and thus a relatively larger tow aspectratio as compared to both twisted tows and UT tows.

Twisting or untwisting can be done either before or after thecarbonization of the precursor filaments as will be apparent to those ofskill in the art. Different procedures can be used. For example,precursor filaments can be carbonized after twisting, and then untwistedthereafter. Alternatively, a precursor filament bundle can be carbonizedin the never twisted condition. Thereafter, twisting can be optimallyperformed on the carbonized tow, as desired.

Another method of arriving at a desired cross sectional shape of a towstrand is by sizing the tow. As is known in the art, sizing refers to aprocess which includes coating or impregnating a tow with a sizingagent, and drying the sizing agent, thus fixing or substantially fixingthe shape of the cross sectional shape of the tow. Any conventionalsizing agents can be used. Preferably, an epoxy-based orepoxy-compatible sizing agent is used. As is known in the art, anepoxy-based sizing agent may optionally contain, e.g., a weak epoxyresin, and additives such as polyethylene glycol, water-solublepolyurethane resin, polyvinyl formal resin, nonionic surfactant and/or acationic surfactant. Various drying methods can be used after the sizingagent is applied, including, e.g., drum drying, air drying, and airblowing. It is noted that drying methods may also affect the crosssectional tow shape and, thus the tow aspect ratio. Typically, sizing isdone after the carbonization process.

In addition, a predetermined, substantially stable cross sectional shapecan be imparted to the tow by passing the tow during the tow preparationprocess, through a specially designed shaping die. The shape of the diecan be designed to be the shape of the desired tow cross section. Priorto contacting the tow with the die, the tow can be subjected to a sizingoperation so that the shaping operation will substantially fix the crosssectional shape of the tow.

In order to achieve a particular fiber tow construction or shape and aparticular fiber tow aspect ratio, different methods can be utilizedindividually or in various combinations. Some minor degree ofexperimentation may be required to determine which method or combinationof methods is most effective, this being well within the capability ofone skilled in the art once apprised of the present disclosure. Forexample, sizing operation can be performed on twisted, NT or UT tows. Inpreparing an UT tow, sizing can be done either before the twisting,after the twisting but before the untwisting, or after the twisting, orin various combinations thereof.

As used herein, “prepreg openness” refers to the percentage of the areain the prepreg which corresponds to the openings in the prepreg, e.g.,openings 26 in FIG. 2. Such openings result from the openings of thefabric used in the prepreg, as indicated by openings 16 in FIG. 1.However, it should be understood that the prepreg openness can bedifferent from the openness of the fabric used in making the prepreg.Typically, when the resin used in the prepreg is transparent, prepregopenness corresponds to the percentage of the area through which lightcan transmit. Thus, the extent of prepreg openness can readily bedetermined by optical inspection and measurements as detailedhereinafter.

Referring back to FIG. 2A, the maximum thickness T_(p) represents themaximum thickness in the area where a warp tow strand 22 intersects andfloats over a weft tow strand 24. Generally, in measuring the thicknessof a prepreg, a thickness gauge with a presser foot covering arelatively large area, e.g., one square inch, is used. As used herein,the term “prepreg thickness” means the average of at least 25-50thickness measurements taken in different representative areas of aprepreg.

For purposes of the present invention, the measurements to determineprepreg thickness, openness, and prepreg tow aspect ratio are takenafter the sample prepregs have been subjected to a conditioningtreatment according to which the prepreg samples are subjected to acompacting pressure of about 45 psi at a temperature of 160° F. forabout three minutes. The compacting pressure is exerted using apneumatic-driven hot press, which contains two parallel platens withheating elements inside controlled by a temperature controller. Theup-and-down movement of these two platens is driven by air pressurecontrolled by an air regulator. This conditioning treatment is performedin order to generate uniform characteristics of the prepreg beingtested. In addition, it is intended to simulate the conditions that aprepreg is subjected to during the curing process for making a fiberreinforced composite structure.

In accordance with the present invention, it has been discovered thatwhen a prepreg is used in a fiber reinforced composite structure, anumber of prepreg properties are related to the degree of core crush ofthe composite structure. In particular, it has been discovered thatthree properties of a prepreg, namely the prepreg thickness, prepregopenness and the average tow aspect ratio of a prepreg, eitherindividually or in combination, are highly determinative the degree ofcore crush. While not wishing to be bound by any theory, it is believedthat such prepreg properties contribute to the roughness of the prepregsurface and thus the friction force between prepreg plies in the lay-uplaminates during the curing process for making a composite sandwichstructure. An increase in roughness and thus friction force leads to thereduction in the degree of core crush. As used herein, the “degree ofcore crush” is defined as the percentage of the area in the sandwichstructure crushed during the curing process as discussed below in detailin connection with the method for testing a prepreg in a core crushpanel.

In particular, it has been found that for prepregs made with fabricshaving the same areal weight, i.e., basis weight, when the prepregthickness is greater, the degree of core crush is lower. It has alsobeen discovered that when the prepreg openness is increased, the degreeof core crush is reduced. Further, the smaller the average fiber towaspect ratio of a prepreg, the lower the degree of core crush that isobserved.

Thus, in accordance with the first embodiment of the present invention,a prepreg is provided which when used in a fiber reinforced compositestructure, greatly reduces the degree of core crush. In particular, whenthe prepreg of this invention is used in making a fiber reinforcedcomposite structure, the degree of core crush is preferably no greaterthan about 15%, more preferably no greater than about 10%, and mostpreferably no greater than about 5%.

In accordance with the first embodiment, the prepreg of this inventionhas a prepreg openness of, at least about 1.0%, preferably at leastabout 2.0%, more preferably at least about 2.5%, even more preferably atleast about 3.0%, and most preferably at least about 3.8% as determinedby the optical inspection method described hereinafter.

However, typically the prepreg openness will be less than about 10.0%,more preferably, less than about 6.0%.

The prepreg has a prepreg thickness of at least about 0.220 mm,preferably at least about 0.245 mm, more preferably at least about 0.250mm, more preferably at least about 0.260, even more preferably at leastabout 0.265 mm, and most preferably at least about 0.270 mm.

Alternatively, the prepreg average tow aspect ratio is less than about15.5, preferably less than about 14.0, more preferably less than about,13.0, even more preferably less than about 12.5, and most preferablyless than about 11.5.

It is noted that in this embodiment of the prepreg of this invention,the prepreg will have at least one of the above-described threeproperties. That is, at least one of the three properties of theprepreg, namely prepreg openness, prepreg thickness, and prepreg towaspect ratio falls with the corresponding range as described above.Preferably at least two of the above three requirements are met in thisembodiment of the invention. Most preferably, all three requirements aremet by the prepreg.

The prepreg in accordance with this first embodiment of the presentinvention can be prepared by the methods as described in detail above.Typically, different fabrics may be impregnated with a resin compositionas described above to prepare a prepreg. The properties of the prepreg,namely, prepreg openness, prepreg thickness, and average tow aspectratio are measured and compared with the above-defined ranges.

In accordance with the second embodiment of the prepreg of thisinvention, a fabric having a complex weave is used in the prepreg toreduce both core crush and porosity. The fabric is of the same type asthe fabric 10 generally described above, i.e., a two-dimensional wovenfabric, preferably a biaxially woven fabric with the warp tow strandscrossing the weft tow strands at a substantially right angle. However,for purpose of this second embodiment of the present invention, the towstrands in one direction, e.g., warp or weft, have a different towconstruction and a different tow aspect ratio, from the tow strands inthe other direction, e.g., weft or warp. By “different tow construction”it is intended to mean that the tow strands in the two directions aredifferent in nature, i.e., they are formed either from different fiberfilaments or by different tow manufacturing processes, or throughmanipulation of the weaving processes, or a combination of theseprocesses. For example, the tow strands in one direction can be twistedtows while the tow strands in the other direction being never twisted(NT) tows. In another example, the tow strands in one direction can havea total filament count of about 3,000 while those in the other directionhaving a total filament count of, e.g., from about 4,000 to about12,000. Since the average tow aspect ratios of the tow strands in thetwo different directions are different in this embodiment of theinvention, the tow strands in the two directions typically havedifferent cross sectional shapes or dimensions. For example, tows in onedirection can be rounder and the tows in the other direction can beflatter, as is apparent from the above detailed description related totow aspect ratio.

While not wishing to be bound by any theory, it is believed that largeopenness may cause a large amount of voids to form in the cured prepregplies of a composite sandwich structure due to air bubbles trappedwithin the plies during the curing process. In addition, it is believedthat when prepreg thickness is increased, porosity may be increased dueto the weave peak/valley mismatches between adjacent prepreg plies. Itis also believed that when prepreg average aspect ratio is too small,prepreg openness can be great enough so as to exacerbate the porosityproblem. It has been found that prepreg openness correlates, to someextent, to porosity, and prepreg openness can be indicative as to thedegree of porosity. Again while not wishing to be bound by any theory,it is believed that, because hybrid tows are employed in the secondembodiment of the present invention, the friction force between theprepreg plies is increased as compared to the conventional prepregsknown heretofore in the art, while the prepreg openness can be kept low.Thus, in a fiber reinforced composite structure prepared with a prepregof this embodiment, both core crush degree and porosity are low.

FIG. 3 illustrates the fabric design of a woven fabric 30, which is anexample of the suitable fabrics for use in the prepreg of the secondembodiment of this invention. FIG. 4 shows a prepreg 40 made byimpregnating fabric 30 with a hardenable resin composition. Fabric 30 isof the same type as fabric 10 shown in FIG. 1. As illustrated in FIGS.3, 4, and FIGS. 4A and 4B, which are cross sectional views along 4A-4Aand 4B-4B in FIG. 4 respectively, the weft tow strands 34 and 44 have adifferent cross sectional tow shape from the warp tow strands 32 and 42.

Referring now to FIG. 4A, in the prepreg 40, the weft tow strands 44have a maximum tow width W_(weft) and a maximum tow thickness T_(weft).Therefore, weft tow strands 44 have a fiber tow aspect ratio W

weft/T_(weft). Likewise, as shown in FIG. 4B, warp tow strands 42 have amaximum tow width WWarp and a maximum tow thickness T_(warp), and thus afiber tow aspect ratio W_(warp)/T_(warp).

In accordance with this aspect of the present invention, the average towaspect ratio of the tow strands in one direction is no greater thanabout 13.0, preferably no greater than about 12.5, more preferably nogreater than about 12.0, most preferably no greater than about 11.0,while the average tow aspect ratio of the tow strands in the otherdirection being at least about 13.5, preferably at least about 14.0,more preferably at least 14.5, and most preferably at least about 15.5.

In addition, the prepreg according to the second embodiment preferablyhas a prepreg openness of no greater than about 5.0%, more preferably nogreater than about 4.0%, even more preferably no greater than about3.5%, and most preferably no greater than about 3.0%.

It is also preferred that the prepreg of this second embodiment has aprepreg thickness of from about 0.230 mm to about 0.300 mm, preferablyfrom about 0.240 mm to about 0.290 mm, more preferably from about 0.250mm to about 0.280 mm, most preferably from about 0.260 mm to about 0.270mm.

In this embodiment of this invention, the fiber tow strands in both warpand weft directions can have the same filament count in each fiber towstrand. Alternatively, the filament counts for the tow strands in twodirections can be different, e.g., about 3,000 in one direction whichhas the lower average tow aspect ratio, and above 3,000 but no greaterthan about 18,000, preferably no greater than about 12,000 in the towstrands having the higher average tow aspect ratio. When the fiberfilament counts are different in the two directions, preferably the towstrands having a greater tow aspect ratio have the greater fiberfilament count.

As discussed above, there are different methods to manipulate fiber towstrands to arrive at the desired tow constructions and tow aspectratios, all of the methods being applicable in this aspect of thepresent invention.

Unexpectedly, when this second embodiment of the prepreg of thisinvention is used in making a fiber reinforced composite structure, boththe degree of core crush and the porosity can be low. Typically, thedegree of core crush is less than about 15%, preferably less than 10%,and more preferably less than 5%. In the meantime, the porosity in thecomposite structure is low and is substantially within the satisfactoryrange for aviation use.

In accordance with yet another embodiment of the present invention,there is provided a prepreg which comprises in the resin-impregnatedfabric, tow strands having a predetermined substantially stable,non-round cross sectional shape. Typically such tow strands have anaverage fiber tow aspect ratio of from about 8.0 to about 18.0,preferably from about 10.0 to about 16.0, even preferably from about12.0 to about 14.5, more preferably from about 12.5 to about 14.0, evenmore preferably from about 13.0 to about 14.0, and most preferably fromabout 13.0 to about 13.5. In addition, the prepreg openness ispreferably no greater than about 5.0%, more preferably no greater thanabout 4.0%, even more preferably no greater than about 3.0%, and mostpreferably no great than about 2.0%. Typically, the prepreg has aprepreg thickness of from about 0.240 mm to about 0.300 mm, preferabyfrom about 0.250 mm to about 0.275 mm, more preferably from about 0.255mm to about 0.270 mm, and most preferably from about 0.260 mm to about0.265 mm. It is noted that the prepreg properties can vary within theabove ranges with tow strands having different filament counts. Forexample, if tows having a greater filament count, e.g., 12,000 areutilized, it is expected that the preferred prepreg thickness would begreater, e.g., at least 0.280 mm, and that the preferred openness wouldbe, e.g., about 4.0%. It has been discovered that the prepreg inaccordance with this third embodiment of the present invention cansubstantially reduce both core crush and porosity in fiber reinforcedcomposite structures.

By “non-round cross sectional shape” it is intended to mean that thecross sectional shape of the tow strand is not round in that thecross-section includes one or more tip ends that taper to a generallypointed end, as opposed to the rounded ends and curved tapering of anoval or similar continuous round shape. Such non-round cross-sectionallows the tow to be sufficiently wide in shape and yet sufficientlythick near the tow centerline to achieve a high surface friction whilereducing openness. Thus, two requirements should be met: first, in agiven tow strand having a fixed total cross sectional area, the towstrand should be made such that a desired average tow aspect ratiowithin the above-described ranges is achieved. Second, the crosssectional tow shape should be arranged such that the greatest crosssectional width of the tow strand is substantially achieved while thedesired average tow aspect ratio is complied with. Generally, so long asthese two requirements are met, the cross section of the tow can be inany non-round shape. For example, as illustrated in FIG. 2C, the crosssectional shape of the tow strand 22′ can be in an eye shape or spearshape, i.e., the two lateral tow tips along the width of the tow aresubstantially extended in the lateral direction, and are preferablysharply pointed ends, while the tow thickness at the center isnevertheless maintained at a certain desired level so as to meet the towaspect ratio requirement as described above. To give another example,the cross sectional tow shape can be in a diamond shape. It is notedthat the perimeter or circumference of the cross section of the towstrand need not be substantially smooth.

The cross sectional tow shape should be substantially stable. In otherwords, once the tow strands are formed, the cross sectional shape of thetow strands are substantially set and are maintained in thesubstantially same shape during the subsequent processes of, e.g.,fabric preparation from tows, prepreg manufacturing from the fabric, thepost-impregnation treatment of the prepreg, as well as the steps in thepreparation of composite structures from the prepreg.

As is apparent from the discussion above in relation to the methods formodifying tow constructions and tow aspect ratio, many differentmethods, either individually or in various combinations, can be used toprepare a tow strand having a predetermined substantially stablenon-round cross sectional shape. Without repeating the details asdescribed above, such methods include, but are not limited to, extrusiontow strands through a specially designed die having the same shape asthe desired cross sectional shape of the tow, sizing the tows prior toweaving, tow twisting, tow twisting and untwisting, varying the crosssectional shapes of the individual filaments in the tows, and variousother modifications of the tow forming process.

Except for the special tow constructions and the prepreg opennessrequirement as specified above, the prepreg according to this embodimentis of substantially the same type as the prepreg illustrated in FIG. 2.FIG. 2C illustrates a cross sectional view of a prepreg in accordancewith this third embodiment of the invention. It is noted that, ascompared to the tow strands of the embodiment shown in FIG. 2B, whilethe tow thickness in FIG. 2C is comparable to that shown in FIG. 2B, thetow width of the tows in this third embodiment is substantially greaterthan that shown in FIG. 2B. As a result, the prepreg openness is reducedwhile the prepreg thickness is not substantially decreased.

In accordance with yet another embodiment of this invention, there isprovided a method for evaluating the core crush resistance properties ofa prepreg for use in a fiber reinforced composite structure. Asdescribed above, it has been discovered in accordance with thisinvention that the prepreg thickness, average tow aspect ratio, andprepreg openness of a prepreg used in the curing process for making afiber reinforced composite structure all correlate with the degree ofcore crush during the curing process. Accordingly, the method forevaluating the core crush resistance property of a prepreg includesdetermining the prepreg thickness, the average tow aspect ratio and/orprepreg openness, and comparing the obtained results to a set ofpredetermined values. Typically, values for prepreg thickness, averagetow aspect ratio, and/or prepreg openness corresponding to differentcore crush degrees are obtained with different types of fabrics and/orresins. Such values can then be used as “predetermined values.” Bycomparing the values for prepreg thickness, average tow aspect ratio,and/or prepreg openness measured from a particular prepreg beingevaluated to the predetermined values, the range within which the degreeof core crush in a curing process using the prepreg will likely fall canbe predicted. It is noted that the predetermined values can vary whensubstantially different fabrics and/or resins are used in preparing theprepreg. By “substantially different fabrics” it is intended to mean,e.g., substantially different areal weights are used, or made fromdifferent types of fibers having substantially different mechanicalproperties, etc. However, an ordinarily skilled artisan apprised of thepresent invention should be able to determine the “predetermined values”for prepregs made from any types of fabrics and/or resins.

By way of example, for prepregs made from woven fabrics having an arealweight of from about from about 150 to about 400 grams per square meter,preferably from about 150 to about 250 grams per square meter, morepreferably from about 180 to about 205, most preferably from about 185to 201 grams per square meter, and consisting essentially of carbonfiber tow strands impregnated with a hardenable resin, a set ofpredetermined values have been determined. The values for prepregthickness and average fiber tow aspect ratio and the correspondingdegree of core crush are shown in FIGS. 5 and 6.

In FIGS. 5 and 6, plain weave fabrics having an areal weight in therange from about 185 to about 201 grams per square meter were preparedfrom different types of carbon fiber tows. The fabrics were impregnatedwith either of the two resin compositions described in Example 1 below.Partly because different fiber tows and different resins were used, theprepregs thus prepared had different prepreg thickness and average towdifferent aspect ratios. As illustrated in FIGS. 5 and 6, both prepregthickness and average tow aspect ratio correlate with the degree of corecrush. In FIGS. 5 and 6, ST stands for a prepreg made from a fabricprepared from carbon fiber tows which have a filament count of 3,000 andare twisted for 15 turns per meter; NT stands for a prepreg made from afabric prepared from carbon fiber tows which have a filament count of3,000 and are never twisted; UT stands for a prepreg made from a fabricprepared from carbon fiber tows which have a filament count of 3,000 andare twisted for 15 turns per meter before sizing and untwisted for 15turns per meter after sizing; Hybrid ST/NT stands for a hybrid prepregas described above prepared from a fabric having ST fiber tows in onedirection and NT fiber tows in the other direction.

As an example, in a prepreg being evaluated, if the average fiber aspectratio is no greater than about 13.5 and the prepreg thickness is morethan about 0.260 nun, the core crush degree in a curing process usingthe prepreg can be predicted to be below about 10%. In addition, it hasalso been determined that when the prepreg openness is greater thanabout 3%, the core crush degree is normally below about 10%.

In the method of the present invention, although a value of only one ofthe three properties, namely prepreg average tow aspect ratio, prepregthickness, prepreg openness, may be sufficient for predicting the corecrush degree, it is preferred that the values for at least two of thethree properties are determined and compared to the predeterminedvalues, preferably one of the two being the prepreg average tow aspectratio. More preferably, the values for all three properties aredetermined and compared to the predetermined values respectively.Although certain discrepancies may result from this method, generallythe accuracy of the prediction based on this method can be above about80%, especially when all three properties are examined. Thus, the methodaccording to the present invention can be very useful in selecting corecrush resistant prepregs for preparing fiber reinforced compositestructures, especially those for aviation use.

In accordance with yet another embodiment of the present invention, afiber reinforced composite structure is provided which is prepared usinga prepreg of this invention as disclosed above. Fiber reinforcedcomposite structures are well known in the art. Different methods forreducing core crush, e.g., various tiedown methods are known in the artin making the present reinforced composite structure of this inventionusing the prepreg provided in the present invention, advantageously theprior art methods and devices for reducing core crush can be omitted,and yet a core crush degree of less than about 15%, more preferably lessthan about 10%, and most preferably less than about 5% can be achieved.Of course, if it is desirable, those prior art devices, such as tiedowndevices, for reducing core crush can also be used in making the fiberreinforced composite structure of this invention.

The fiber reinforced composite structure can be prepared by any suitablemethods known in the art. Typically, plies of the prepreg of thisinvention are laid up in laminates on one or both sides of a lightweightcore or honeycomb core formed of e.g., aluminum, Nomex®, fiberglass,etc. The lay-up is then autoclaved in a vacuum bag placed in anautoclave under conditions such that the prepregs are cured and adheredto the honeycomb core. For example, U.S. Pat. No. 5,685,940, which isincorporated herein by reference, discloses an improved method formaking a fiber reinforced composite structure, which can be used in thepresent invention.

The invention is further demonstrated in the following examples, whichare used only for purpose of illustration but not to limit the scope ofthe present invention.

In accordance with the invention, prepreg and composite properties areevaluated using the testing methods described below. It should be notedthat in these methods, the measurements indicated herein for prepregthickness, openness, and prepreg tow aspect ratio are taken after thesample prepregs have been subjected to a conditioning treatmentaccording to which the prepreg sample is subjected to a compactingpressure of 45 psi using a pneumatic-driven hot press at 160° F. forthree minutes. This conditioning treatment is performed in order togenerate uniform characteristics of the prepreg being tested. Inaddition, it is intended to simulate the conditions that a prepreg issubjected to during the curing process for making a fiber reinforcedcomposite sandwich structure.

Method of Measuring Prepreg Openness

A piece of prepreg is laid flat under a microscope with transmittedlight passing through the prepreg from under. No force or pressure isapplied on this prepreg. An image, showing the prepreg and its opennessas black and very light gray respectively, is viewed by a video camera(attached to the microscope) which transmits the image in detail form tothe image grabber of a PC computer. The image is then converted into arectangular array of integers, corresponding to the digitized gray levelof each picture element (pixel). An image analysis program such asOptimas 6.2, in the PC, or the like is used to process this digitalimage information and represents it in the form of a gray levelhistograms. This histograms summarizes the gray level content of theimage. In this case two distinct groups, corresponding to the prepregand openness, can be found in the histograms. These two groups can beeasily separated by a simple thresholding process. The prepreg opennessis thus obtained as the percentage ratio between the number of pixelscorresponding to the group associated with openness and the total numberof pixels in the image.

In order to obtain more accurate and representative results, openness ismeasured at a very low magnification (5× or under). Each image containsat least ten fiber tow strands in each direction. Several pieces ofprepreg randomly chosen from different locations of a prepreg roll aremeasured, and an average is taken as the openness of the prepreg.

Method of Measuring Prepreg Thickness

Typically, prepreg thickness is measured by a thickness gauge withpresser foot covering a relatively large area on the prepreg surface.For example, a test setup similar to ASTM D1777-96 (Standard Test Methodfor Thickness of Textile Materials) is used. The apparatus contains athickness gauge with a one inch square presser foot and a 5 pounds deadweight on it. This is equivalent to approximately 5 psi pressure appliedto the specimen when a measurement is taken. Several pieces of prepregare randomly chosen from different locations of a prepreg roll andseveral measurements are taken for each piece. The average of the allthe measurements from one prepreg roll can be taken as the prepregthickness.

Method of Measuring Fiber Tow Width and Thickness in Prepreg

The width of a fiber tow strand can be determined by thelight-transmission method as described above for determining prepregopenness, except that high magnification is used in this case toincrease the measurement resolution and fiber tow is magnified in such away that its width covers most part of the image. The same image program(Optimas 6.2) is calibrated before measurement through an acquired imageof a very fine ruler positioned at the same height as the measuredobject. By using the Optimas software to draw a line on a knowndimension of the ruler, the programs can “memorize” the length of thisline and use this information as a base for any other lengthmeasurements under the same condition. Fiber tow width is then measuredby simply drawing a line to cover the entire fiber tow width and theprograms can automatically calculate the length of this line based onthe saved calibration data.

Again, several pieces of prepreg are randomly chosen from differentlocations of the prepreg roll and each piece has several measurements.Final number is based on the average of all measurements.

Similarly, to measure the fiber tow thickness, a piece of prepreg iscarefully cut with surgical scissors along the centerline of fiber tows.The thickness is then determined in a method similar to that formeasuring the width of a fiber tow strand. Again, no force or pressureis applied. Very high magnification is necessary in this case toincrease the measurement resolution. Light sources can be positioned insuch a way that the 90-degree layer becomes white in contrast to dark0-degree layer for each measurement. Measurements are made again in bothwarp and fill directions.

Testing Prepregs in Core Crush Panels

A standard core crush panel 70 as shown in FIG. 7 is used consisting of28″×24″ composite skins and a 24″×20″ Nomex core (⅛″ cell size, 0.5″thick, 3.0 pcf, e.g., Hexcel Corporation HRH-10, or equivalent) with a20° chamfer angle. FIG. 7A is a cross section view of the core crushpanel 70, illustrating the lay up of the core crush panel prior to beingcured. The types, directions, and dimensions of prepreg plies, as wellas those of the honeycomb core are specified in Table I. Additionally,layers of adhesive are typically between the core and prepreg plies 75and 77, and below ply 72 (not shown).

TABLE I PLY NUMBER TYPE DIRECTION DIMENSION 72 FULL 0°-90° 28″ × 24″ 73DOUBLER ±45° 28″ × 24″ Picture-frame opening to allow 1.25″ back fromcore mold point 74 DOUBLER 0°-90° 28″ × 24″ Picture-frame opening toallow 0.75″ back from core mold point 75 FULL ±45° 28″ × 24″ CORETRANSVERSE 20″ × 24″ 2.0″ upper edge radius angle radius fair from 2.0″to 0.15″ 76 FILLER 0°-90° 2 Pcs-2″ × 26″ 2 Pcs-2″ × 24″ 77 FULL ±45° 2825″ × 24.25″ 78 DOUBLER 0°-90° 28.25″ × 24.25″ Picture-frame opening toallow 0.75″ back from core mold point 79 DOUBLER ±45° 28.25″ × 24.25″Picture-frame opening to allow 1.25″ back from core mold point 80 FULL0°-90° 28.25″ × 24.5″

To cure the panel 70, the panel is placed in a vacuum bag. The vacuumbag and the panel therein are then placed in an autoclave. The bag isevacuated and cured under pressure at an elevated temperature. Thecuring cycle includes the following steps: (1) applying vacuum of 3.9psia (27 kPa) minimum to the vacuum bag; (2) pressurize the autoclave to413 kPa (45 psia) (including venting the vacuum bag to atmosphere whenthe autoclave pressure reaches 20 psia); (3) raising the temperatureinside the autoclave at a rate of 1-5° F./min; (4) curing the panel at355° F. for 2 hours (under the pressure established in step 2); (5)cooling down at a rate of 5° F./min, and (6) following curing, when parttemperature has fallen to 140° F., relieving the pressure, removing thevacuum bag and debagging.

The dimensions of the cured core crush panel are measured as shown inFIG. 8. X is the displacement of the center of the core side from itsoriginal position. L represents the original length of core side. Thecrushed area A is calculated according to the formula:$A = {\sum\limits_{n = 1}^{4}{{2/3} \cdot X_{n} \cdot L_{n}}}$

The degree of core crush in percentage is determined by the followingformula:

Percent core crush=100×A/480.

To examine interlaminar porosity, a standard core crush panel is cutalong the line marked 7A—7A in FIG. 7. The cross section is examinedvisually. One linear inch of the exposed edge in a ply area whichappears to have the highest porosity is polished with a diamond polish(e.g., 0.3-micron diamond polish) and examined under 50× magnificationfor internal porosity. Five specimens are measured and an average istaken as the porosity.

In the following example, prepregs were made in accordance with thepresent invention. The sample prepregs were subjected to a conditionaltreatment: a compaction pressure of 45 psi was applied on the sampleprepregs at 160° F. for three minutes to partially simulate thecompaction imported to the panel in the autoclave during the curingprocess. The prepreg openness, thickness, and fiber tow aspect ratiowere then measured from the sample prepregs. The prepregs were tested ina core crush panel for their core crush resistance and porosity.

EXAMPLE

Fabrics were prepared from ST (standard twisted), UT (untwisted), NT(never twisted) fiber tows, or both ST and NT fiber tows (Hybrid ST/NT,i.e., ST in warp direction and NT in weft direction) respectively. Asused herein, ST tows stand for carbon fiber tows which were twisted for15 turns per meter during the manufacturing process; NT tows are carbonfiber tows which were never twisted during the tow manufacturingprocess; UT tows are carbon fiber tows which were twisted for 15 turnsper meter before sizing and untwisted for 15 turns per meter aftersizing. Each tow had a total carbon fiber filament count of about 3,000.All fabrics used were plain weave with a weaving pitch of 12-13tows/inch in both warp and weft directions, and had a fabric arealweight of about 193 grams per square meter.

The fabrics were impregnated by a solution method with either of thefollowing two epoxy resin compositions: Resin No. 1 comprises of about67% multifunctional epoxy resins, about 8.3% solid and liquid reactiveelastomers, about 20.7% of a multifunctional amine curing agent, about1.8% of a co-curing agent, 0.1% of a catalyst, and 2.1% of a flowcontrol agent, i.e., fumed silica. The resin composition No. 1 has a tanδ of 0.78 at 70° C. and 0.27 at 140° C.

Resin No. 2 comprises of about 67.6% multifunctional epoxy resins, about7.4% solid and liquid reactive elastomers; 20.7% of a multifunctionalamine curing agents; about 1.8% of a co-curing agent; 0.1% of acatalyst, and 2.1% of a flow control agent, i.e., fumed silica. Theresin composition No. 2 has a tan δ of 1.37 at 70° C. and 1.35 at 140°C.

The prepregs thus prepared were measured for prepreg thickness,openness, and fiber tow aspect ratio. Core crush panels were preparedfrom the prepregs and core crush degree and porosity were determined.The results are shown in Table II. Porosity is indicated qualitativelyin a scale of 1 to 5, where 1 represents a low level while 5corresponding to a high level.

TABLE II Prepreg Prepreg Core Resin Thickness Openness Fiber Tow AspectRatio Crush Fabric composition (mm) (%) Warp Weft Average (%) PorosityST (comparative) No. 1 0.289 4.9 8.8 11.9 10.4 0.73 5 ST No. 2 0.260 3.110.7 15.4 13.1 5.0 1-2 UT (comparative) No. 1 0.277 1.9 11.4 12.5 120.59 3 NT (comparative) No. 1 0.244 1.7 13.5 16.2 14.9 16 1 NT(comparative) No. 2 0.224 1.7 15.5 18.5 17 22.3 1 Hybrid ST/NT No. 10.272 3.8 9.7 15.5 12.6 3.4 3

As shown in Table II, when the prepreg thickness, average tow aspectratio, and prepreg openness meet the requirements of this invention, thedegree of core crush generally is low, i.e., lower than about 15%. Forexample, prepregs prepared from fabrics made from ST or UT fiber towsmeet the prepreg thickness, average tow aspect ratio, and prepregopenness requirements, while those prepared from fabrics made from NTfiber tows do not meet the requirements. Consequently, as shown in TableII, the core crush degree in a fiber reinforced composite sandwichstructure prepared using the former prepregs is substantially lower thanthat in a fiber reinforced composite sandwich structure prepared usingthe latter prepregs. Although prepregs made from plain weave fabricsprepared from ST, UT, and NT fiber tows having a filament count of 3,000and impregnated with a conventional epoxy resin such as Resin No. 1 wereknown, the fact that prepreg openness, prepreg thickness and average towaspect ratio are highly determinative of the core crush degree has neverbeen appreciated. Accordingly, those skilled in the art prior to thisinvention generally expected the different tows to perform comparably.

In addition, while certain prepregs, e.g., prepregs prepared fromfabrics made of ST tows, can reduce the degree of core crush, they areusually associated with unacceptable porosity. (See ST with Resin No. 1in Table II.) Unexpectedly, when an epoxy resin such as the Resin No. 2is used which has a preferred tan δ value and an average functionalityof greater than 2, the porosity in the fiber reinforced compositesandwich structures prepared from the prepregs made from ST fabrics canbe significantly reduced to an acceptable level, as shown in Table II.It is also believed that when the same resin is applied to a fabric madefrom UT tows, the porosity will be reduced as well. However, the use ofa resin such as Resin No. 2 does not reduce core crush degree. Forexample, as shown in Table II, while the prepreg prepared from a fabricmade from NT tows impregnated with Resin No. 1 causes unacceptable corecrush, the same fabric impregnated with Resin No. 2 is associated withno less core crush.

Further, when a prepreg prepared from a hybrid fabric having ST tows inone direction and NT tows in the other direction is used, both corecrush degree and porosity are satisfactory even when a conventionalresin such as Resin No. 1 is used.

Thus, as demonstrated by the Example, the present invention providesprepregs which when used in making fiber reinforced compositestructures, can effectively reduce both core crush and porosity. Inaddition, because various fiber tows can be manipulated to impartdesirable prepreg properties so as to reduce core crush, this inventionallows prepreg manufacturers greater freedom in selecting fiber tows orfabrics and thus less dependence on specific specialty fiber towmanufacturers.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

That which is claimed:
 1. A fiber reinforced composite structure,comprising: a laminate adhered to a core, said laminate being formedfrom at least one core crush resistant prepreg, said prepreg comprising:a woven fabric consisting essentially of carbon fiber tow strandsimpregnated with a hardenable polymeric resin composition, said fabrichaving an areal weight of from about 150 to about 400 grams per squaremeter, said resin composition having an average epoxy functionality ofgreater than 2.0, and a tan δ of between about 1.0 and about 2.0 at 70°C., between about 0.7 and about 2.0 at 100° C., and between about 0.5and about 2.0 at 140° C. or the highest temperature of minimum resinviscosity; and said prepreg having an average fiber tow aspect ratio ofless than about 15.5 , a prepreg thickness of at least about 0.245 mm,and a prepreg openness of from about 1.0% to about 10%.
 2. The fiberreinforced composite structure of claim 1, wherein said prepreg has anaverage fiber tow aspect ratio of less than about 13.0.
 3. The fiberreinforced composite structure of claim 1, wherein said prepreg has aprepreg thickness of at least about 0.260 mm.
 4. A fiber reinforcedcomposite structure, comprising: a laminate adhered to a core, saidlaminate being formed from at least one core crush resistant prepreg,said prepreg comprising: a woven fabric impregnated with a hardenablepolymeric resin composition, said woven fabric having an areal weight offrom about 150 to about 400 grams per square meter and consistingessentially of a plurality of carbon fiber tow stands in a firstdirection interweaving with a plurality of carbon fiber tow strands in asecond direction, said tow strands in said first direction having adifferent tow construction from said tow strands in said seconddirection, wherein said prepreg has a tow aspect ratio of no greaterthan about 13.0 in said first direction and a tow aspect ratio of atleast 13.5 in said second direction, and a prepreg openness of nogreater than 4.0%.
 5. The fiber reinforced composite structure of claim4, wherein said tow strands in said first direction are twisted tows andsaid tow strands in said second direction are never twisted.
 6. Thefiber reinforced composite structure of claim 4, wherein said resincomposition comprises an epoxy resin having an average epoxyfunctionality of greater than 2.0 and a tan δ of from about 0.5 to about2.0 throughout the elevated temperature range of from about 70 to about140° C.
 7. A fiber reinforced composite structure comprising: a laminateadhered to a core, said laminate being formed from at least a core crushresistant prepreg, said prepreg comprising: a woven fabric consistingessentially of carbon fiber tow strands impregnated with a hardenablepolymeric resin composition, said tow strands having a predeterminedsubstantially stable non-round cross-sectional shape, said fabric havingan areal weight of from about 150 to about 400 grams per square meter,said prepreg having an average tow aspect ratio of from about 12.0 toabout 14.5, and a prepreg openness of no greater than about 4.0%.
 8. Thefiber reinforced composite structure of claim 7, wherein said crosssectional shape is a diamond shape.
 9. The fiber reinforced compositestructure of claim 7, wherein said cross sectional shape is a spearshape having two pointed ends.
 10. A fiber reinforced compositestructure, comprising: a laminate adhered to a core, said laminate beingformed from at least one core crush resistant prepreg, said prepregcomprising: a woven fabric having a plain weave pattern consistingessentially of carbon fiber tow strands impregnated with a hardenablepolymeric resin composition comprising an epoxy resin; said fabrichaving an areal weight of from about 180 to about 205 grams per squaremeter; each of said tow strands having a total filament count of about3,000; said resin composition having an average epoxy functionality ofgreater than 2.0, and a tan δ of from about 0.5 to about 2.0 throughoutthe elevated temperature range of from about 70 to about 140° C.; andsaid prepreg having an average fiber tow aspect ratio of less than about13.0, and a prepreg openness of from about 1.5% to about 10.0%.
 11. Afiber reinforced composite structure, comprising: a laminate adhered toa core, said laminate being formed from at least one core crushresistant prepreg, said prepreg comprising: a woven fabric having aplain weave pattern consisting essentially of carbon fiber tow strandsimpregnated with a hardenable polymeric resin composition comprising anepoxy resin; said fabric having an areal weight of from about 180 toabout 205 grams per square meter; each of said tow strands having atotal filament count of about 3000; said resin composition having anaverage epoxy functionality of greater than 2.0, and a tan δ of fromabout 0.5 to about 2.0 throughout the elevated temperature range of fromabout 70 to about 140° C.; and said prepreg having an average fiber towaspect ratio of less than about 13.0, a prepreg thickness of at leastabout 0.260 mm, and a prepreg openness of from about 1.0% to about 10%.12. A fiber reinforced composite structure, comprising: a laminateadhered to a core, said laminate being formed from at least one corecrush resistant prepreg, said prepreg comprising: a woven fabricconsisting essentially of carbon fiber tow strands impregnated with ahardenable polymeric resin composition; said fabric having an arealweight of from about 150 to about 250 grams per square meter; said towstrands having a total filament count of from about 1,000 to less thanabout 3,000; and said prepreg having an average fiber tow aspect ratioof less than about 15.5, an average thickness of at least about 0.220mm, and an openness of from about 1.0 percent to about 6.0 percent. 13.The fiber reinforced composite structure of claim 12, wherein said resinhas an average epoxy functionality of greater than 2.0, and a tan δ offrom about 0.5 to about 2.0 throughout the elevated temperature range offrom about 70 to about 140° C.
 14. A fiber reinforced compositestructure, comprising: a laminate adhered to a core, said laminate beingformed from at least one core crush resistant prepreg, said prepregcomprising: a woven fabric consisting essentially of carbon fiber towstrands impregnated with a hardenable polymeric resin composition; saidfabric having an areal weight of from about 150 to about 400 grams persquare meter; said tow strands having a total filament count of greaterthan about 3,000 but no greater than about 18,000; and said prepreghaving an average thickness of at least about 0.260 mm and an opennessof at least about 1.5 percent.
 15. The fiber reinforced compositestructure of claim 14, wherein said resin composition comprises an epoxyresin having an average epoxy functionality of greater than 2.0 and atan δ of from about 0.5 to about 2.0 throughout the elevated temperaturerange of from about 70 to about 140° C.
 16. A fiber reinforced compositestructure comprising: a laminate adhered to a core, said laminate beingformed from at least one core crush resistant prepreg, said prepregcomprising: a woven fabric impregnated with a hardenable polymeric resincomposition; said woven fabric having an areal weight of from about 150to about 400 grams per square meter and consisting essentially of aplurality of carbon fiber tow strands in a first direction interweavingwith a plurality of carbon fiber tow strands in a second direction, saidtow strands in said first direction having a different tow constructionfrom said tow strands in said second direction; wherein said prepreg hasa tow aspect ratio of no greater than about 13.0 in said first directionand a tow aspect ratio of at least 13.5 in said second direction, and aprepreg openness of no greater than 5.0%.
 17. The fiber reinforcedcomposite structure of claim 16, wherein said tow strands in said firstdirection and said tow strands in said second direction have a fiberfilament count of about 3,000.
 18. The fiber reinforced compositestructure of claim 16, wherein said tow strands in said first directionhave a fiber counts of about 3,000 and said tow strands in said seconddirection have a fiber filament count of about 6,000.
 19. The fiberreinforced composite structure of claim 16, wherein said tow strands insaid first direction are twisted tows and said tow strands in saidsecond direction are never twisted.
 20. The fiber reinforced compositestructure of claim 16, wherein said tow strands in said first directionare untwisted tows and said tow strands in said second direction arenever twisted.
 21. The first reinforced composite structure of claim 16,wherein said resin composition comprises an epoxy resin.
 22. The fiberreinforced composite structure of claim 16, said resin composition hasan average epoxy functionality of greater than 2.0, and a tan δ of fromabout 0.5 to about 2.0 throughout the elevated temperature range of fromabout 70 to about 140° C.
 23. A fiber reinforced composite structure,comprising: a laminate adhered to a core, said laminate being formedfrom at least one core crush resistant prepreg, said prepreg comprising:a woven fabric impregnated with a hardenable liquid epoxy resincomposition, said woven fabric having an areal weight of from about 185to about 205 grams per square meter and consisting essentially of aplurality of carbon fiber tow strands in a first direction interweavingwith a plurality of carbon fiber tow strands in a second direction, saidtow strands in said first direction having a different tow constructionfrom said tow strands in said second direction, wherein said prepreg hasa tow aspect ratio of no greater than about 11.0 in said first directionand a tow aspect ratio of at least 15.5 in said second direction, aprepreg openness of no greater than 3.0%, and a prepreg thickness offrom about 0.250 mm to about 0.280 mm.
 24. A fiber reinforced compositestructure, comprising: a laminate adhered to a core, said laminate beingformed from at least one core crush resistant prepreg, said prepregcomprising: a woven fabric consisting essentially of carbon fiber towstrands impregnated with a hardenable polymeric resin composition, saidtow strands having a predetermined substantially stable non-round crosssectional shape; said fabric having an areal weight of from about 150 toabout 400 grams per square meter; said prepreg having an average towaspect ratio of from about 8 to about 18, and a prepreg openness of nogreater than about 5.0%.
 25. The fiber reinforced composite structure ofclaim 24, wherein said prepreg has a prepreg average thickness of fromabout 0.240 mm to about 0.300 mm.
 26. The fiber reinforced compositestructure of claim 24, wherein said cross sectional shape is a diamondshape.
 27. The fiber reinforced composite structure of claim 24, whereinsaid cross sectional shape is a spear shape having two pointed ends. 28.The fiber reinforced composite structure of claim 24, wherein each ofsaid tow strands has a total fiber filament count of from about 1,000 toabout 18,000.
 29. The fiber reinforced composite structure of claim 24,wherein said fabric has a plain weave pattern.
 30. The fiber reinforcedcomposite structure of claim 24, wherein said fabric has a satin weavepattern.
 31. The fiber reinforced composite structure of claim 24,wherein said resin composition comprises a multifunctional epoxy resinhaving an average epoxy functionality of greater than 2.0, and a tan δfrom about 0.5 to about 2.0 throughout the elevated temperature range offrom about 70 to about 140° C.
 32. A fiber reinforced compositestructure, comprising: a laminate adhered to a core, said laminate beingformed from at least one core crush resistant prepreg, said prepregcomprising: a woven fabric consisting essentially of carbon fiber towstrands impregnated with a hardenable polymeric resin compositioncomprising an epoxy resin, said tow strands having a predeterminedsubstantially stable non-round cross sectional shape and a filamentcount of about 3,000; said fabric having an areal weight of from about180 to about 205 grams per square meter; said prepreg having an averagetow aspect ratio of from about 12.5 to about 14.0, a prepreg openness ofno greater than about 3.0%, and a prepreg thickness of from about 0.250mm to about 0.280 mm.